Antennas for mobile or portable use should be small, light weight, rugged in construction, pleasing in appearance and low in cost. However, more importantly, the antenna must be able to perform as a receiver and transmitter of radio frequency signals within the mobile power source limitations at a high omnidirectional gain in a difficult environment.
Typical mobile transceivers currently employ quarter-wave whip antennas (see: Standard Handbook for Electrical Engineers, Tenth Edition, by Donald G. Fink and John M. Carroll, editors, McGraw-Hill, 1968, New York, page 25-74). A fairly uniform omnidirectional vertical polarity pattern is obtained from such installations. However, these antennas require significant space, distance from other conductive materials, specific position with respect to the environment and are usually placed above a horizontal plane.
Various other antenna techniques and structures are also known. These include: employing conducting and non-conductive portions of the mobile structure (see for example: U.S. Pat. Nos. 4,317,121; 4,160,977; 4,117,490; 3,961,330; and 3,916,413); bonding the antenna structure to the non-conductive portions of the mobile structure (see for example: U.S. Pat. Nos. 4,331,961; and 3,646,561); embedding the antenna or caged antenna in the mobile structure (see for example: U.S. Pat. No. 3,717,876) and reducing the dimensions to a small fraction of the wavelength. These approaches typically depend upon added nonconductive material, typically air, as a dielectric to insulate the conductive antenna elements. A final approach is to use a dipole element for the conductive portions.
The use of dipole elements in an antenna can be as simple as a straight radiator fed in the center to produce currents with two nodes, one at each of the far ends of the radiator (see Van Nostrand's Scientific Encyclopedia, Fourth Edition, D. Van Nostrand Company, Princeton, N.J., 1968, Page 537). Analysis of the field intensity of these elementary dipole antennas is segregated into short distance (less than 0.01 wavelengths), intermediate (0.01 to 5.0 wavelengths) and great distance (greater than 5 wavelengths), see Reference Data for Radio Engineers, Fourth Edition, Published by International Telephone and Telegraph Corporation, New York, 1956, pages 662-665. The two nodes are typically insulated from each other (except at the central point/area of connection) by air. In order to improve tuning and balance, various geometries are used. Two separate radiator elements can also be used. Variations with two separate radiator elements include: slots, altering sizes of nodes, folded radiators and adding/altering the dielectric between the elements (see the section on Slot Antennas, specifically the relationship to metallic dipole antennas, supra, pages 687-689, and U.S. Pat. No. 3,210,766).
Small sizes of antenna are particularly desirable for mobile application, as space and wind resistance consideration may be critical. Patch or microstrip antennas have been developed for this application (see Micro-Strip Antennas, 2nd Edition by Bahl & Bhartia, published by Artech House, at Ottawa, Canada, 1982, Page 27). These typically provide a first element (top hat) and second element (ground plane) which sandwich a dielectric material, feed by a coaxial cable. This type of antenna is currently used for cellular communications over the 822-890 MHZ frequency band. This approach produces a very small package, but with limitations.
Limitations of these patch antennas are primarily related to the narrow band of performance and the poor gain produced within that narrow band. Typical gains are in the order of zero to negative 2 Dbd from a standard dipole reference over the same band of frequencies. Frequency band for these "gains" is typically limited to the order of 40 MHZ (less than the entire bandwidth form 822 to 890 MHZ). Other limitations include the sensitivity to other dielectrics proximate to the radiating elements in the environment and exposure of the (conductive) elements, requiring additional protection from shorting or damage.
An additional limitation is related to the unbalance caused by the coaxial cable coupling with respect to the radiating elements, and the unsymmetrical geometry of the antennas (see Transmission Lines Antennas and Wave Guides, First Edition, by Ronold W. P. King, Harry Rowe Mimno, and Alexander H. Wing, Published by McGraw-Hill Book Company, 1945, Pages 130-133, 145-149). A coaxial line parallel and feeding the antenna can also have antenna currents, that is currents excited by the antenna. In a metal shield of a coaxial cable, antenna currents may be primarily on the outer surface of the outer conductor. At high frequencies this can cause the coaxial line to act as a three conductor (outer and inner surface currents on the outer conductor as well as transmission currents on the inner conductor). Asymmetrical geometries of the coaxial cable, impedance sections and antenna also lead to unwanted antenna currents. Prior art concentrated on the geometry and spacing of the two radiating and coaxial elements to minimize these problems with unwanted antenna currents.